Ensure template is not sent for Structured logs with no parameters

CHANGELOG.md

@@ -1,5 +1,11 @@
 # Changelog
 
+## Unreleased
+
+### Fixes
+
+- Templates are no longer sent with Structured that have no parameters ([#4544](https://github.com/getsentry/sentry-dotnet/pull/4544))
+
 ## 5.15.1
 
 ### Fixes

src/Sentry/Internal/DefaultSentryStructuredLogger.cs

@@ -53,7 +53,9 @@ private protected override void CaptureLog(SentryLogLevel level, string template
 
         SentryLog log = new(timestamp, traceHeader.TraceId, level, message)
         {
-            Template = template,
+            // the SDK MUST NOT attach a sentry.message.template attribute if there are no parameters
+            // https://develop.sentry.dev/sdk/telemetry/logs/#default-attributes
+            Template = parameters is { Length: > 0 } ? template : null,
             Parameters = @params,
             ParentSpanId = traceHeader.SpanId,
         };

test/Sentry.Tests/SentryStructuredLoggerTests.Format.cs

@@ -26,6 +26,40 @@ public void Log_Enabled_CapturesEnvelope(SentryLogLevel level)
         _fixture.AssertEnvelopeWithoutAttributes(envelope, level);
     }
 
+    [Theory]
+    [InlineData(SentryLogLevel.Trace)]
+    [InlineData(SentryLogLevel.Debug)]
+    [InlineData(SentryLogLevel.Info)]
+    [InlineData(SentryLogLevel.Warning)]
+    [InlineData(SentryLogLevel.Error)]
+    [InlineData(SentryLogLevel.Fatal)]
+    public void Log_NoParameters_TemplateNotSet(SentryLogLevel level)
+    {
+        // Arrange
+        _fixture.Options.Experimental.EnableLogs = true;
+        var logger = _fixture.GetSut();
+
+        Envelope envelope = null!;
+        _fixture.Hub.CaptureEnvelope(Arg.Do<Envelope>(arg => envelope = arg));
+
+        // Act
+        logger.Log(level, "Template string without arguments");
+        logger.Flush();
+
+        // Assert
+        _fixture.Hub.Received(1).CaptureEnvelope(Arg.Any<Envelope>());
+        var envelopeItem = envelope.Items.Should().ContainSingle().Which;
+        var log = envelopeItem.Payload.Should().BeOfType<JsonSerializable>().Which.Source.Should().BeOfType<StructuredLog>().Which;
+        log.Should().NotBeNull();
+        var items = log.Items;
+        items.Length.Should().Be(1);
+        var item = items[0];
+
+        // If there are no sentry.message.parameter.X attributes included in the log, then
+        // the SDK MUST NOT attach a sentry.message.template attribute.
+        item.Template.Should().BeNull();
+    }
+
     [Theory]
     [InlineData(SentryLogLevel.Trace)]
     [InlineData(SentryLogLevel.Debug)]